The present invention relates to a driving force distribution control system for controlling a transfer of a 4WD system for distributing a driving force of an engine between front and rear wheels.
One conventional example is shown in Japanese patent provisional publication no. 61-157437. A driving force distribution control system of this conventional example is arranged to calculate a front and rear wheel speed difference representing a drive wheel slip, from sensor signals, and to increases an engagement force of a transfer clutch as the front and rear wheel speed difference increases. Thus, this system can restrain the drive wheel slip by increasing the tendency to 4WD with increase of the drive wheel slip.
However, this conventional control system is arranged to increase the clutch engagement force with increase in the front and rear wheel speed difference, and control constants are set so that the steer characteristic of the vehicle remains neutral steer over an entire vehicle speed range from low and medium speeds to high speeds. Therefore, this conventional system is still problematical in the following points.
(i) Even in a high speed straight ahead operation, the transfer clutch is engaged with a delay after occurrence of the front and rear wheel speed difference. Therefore, the stability is poor especially in an operation for lane change, and to minute disturbances. PA1 (ii) The steer characteristic is always a neutral-steer characteristic whereas a weak understeer characteristic is desired in a high speed turning operation with acceleration. Therefore, the directional control is difficult in a high speed turning operation. PA1 (iii) If the driving force distribution control system is adjusted so as to meet the above-mentioned problems (i) and (ii) in the high speed driving, then the stability at low and medium speeds is deteriorated. It is difficult or impossible to meet both the requirement in the low and medium speed range and the requirement in the high speed range.